1. Field of the Invention
This invention relates to new and useful improvements in threaded tool joints for connecting pipe together and more particularly to a tool joint having low stress, and reduced susceptibility to fatigue failure and to over-torquing during use.
2. Brief Description of the Prior Art
Threaded tool joints for connecting pipe together are commonly used in drilling operations in the petroleum industry. These tool joints consist of a male pin member which makes up into a female box member. One of the major problems with tool joints is the failure of the joint as a result of fatigue after an extended period of use.
"Tool joint", as used herein, is any threaded connection in a drill string used for petroleum or mining purposes, such as intergal or welded on joints, drill collars, stabilizers, reamers, etc.
Previous methods of solving the fatigue problem with threaded tool joints have involved principally the use of surface peening methods and also stress relief grooves introduced near the shoulder of the pin. These methods have some limited success but generally are concerned with the effect of high stress concentrations in such regions as the thread root and shoulder to land interface.
Higbee U.S. Pat. No. 658,087 discloses a tapered threaded coupling which produces a jamming union when coupled with other threaded members.
Van Der Wissel U.S. Pat. No. 3,050,318 discloses a box and pin type threaded joint in which the bearing face of the threads changes along the length of the threaded joint.
Hjalsten U.S. Pat. No. 3,388,935 discloses a threaded drill rod element having a shallow thread which varies in the amount of force applied to the threads along the length thereof.
Miida U.S. Pat. No. 4,113,290 shows a threaded connection for large diameter casing in which the thread taper and the thread profile changes the point of application of pressure along the length of the thread.
Holmberg U.S. Pat. No. 4,295,751 discloses still another threaded connection in which the threads are designed to vary the amount of force applied on the individual threads. This threaded connection is designed for use in a percussion drill.
Over torquing of the tool joint in use is a problem in some formations. It has been suggested to utilize a second pair of shoulders which abut at approximately full make up of the joint to greatly increase the torque needed to over-torque the joint. See McArthur U.S. Pat. No. 2,532,632. This requires careful machining of a second pair of shoulders at close tolerance.